On bonded interface admin-down, should stop all slave links

When bonded interface is set to admin down, all slave links should
also be stopped or they will continue to receive packets and then
be dropped because bonded interface is down.

Also remove setting bonded interface L3 packet size limit to that of
slave on startup - it is not needed and can cause undesirable side
effect if its value become incompatible with bonded interface MTU.

Change-Id: Ibdd8acac0ca41a867131441eb26518592bb7d98a
Signed-off-by: John Lo <loj@cisco.com>